This invention relates to the field of binocular viewing devices. More specifically, it relates to binocular viewing devices used in helmet-mounted displays.
In recent years, a great deal of effort has been spent on developing viable helmet-mounted displays (`HMD`s). HMDs are the probable successor to head-up displays (`HUDs`). In a HUD, information which is normally displayed on cockpit instruments which require the pilot to look inside the airplane to read, compromising the pilot's ability to scan for targets or potential hazards outside the aircraft, is instead collected and displayed on a transparent panel mounted directly in front of the pilot.
Although HUDs certainly improve the situational awareness of pilots, they do have operational problems. The field of view tends to be relatively narrow. A pilot looking off to one side for a target or other aircraft will lose the data displayed on the HUD. Also, very intense sunlight can "wash out" the information displayed on the HUD. Although newer HUDs have mostly overcome this second problem, it is still not completely solved.
HMDs represent an attempt to improve upon HUDs. In very simplified form, an HMD comprises a helmet which incorporates the electronics and optics needed to create the flight symbology and to display this information directly in front of the pilot's eyes, along with other flight related equipment necessary to safeguard the pilot.
By placing the display directly in front of the pilot's eyes, the flight data symbology is not lost when the pilot turns his head, as the display moves with him. Although HMDs offer real advantages, many design problems still exist.
For the display of symbology on the HMD during daytime, it is likely that a relatively small field of view (`FOV`) would suffice. However, for very low-level flight, often called nape-of-the-earth (`NOE`) flight, especially night NOE, synthetic sensor-derived imagery may be needed. A narrow FOV will not suffice for such flight regimes. Unfortunately, an increase in FOV comes with an increase in the size and weight of the optical channels.
To date, HMDs have been used primarily in simulators, and have generated a large FOV. The relatively large size and weight of these systems is of little concern, as there is no requirement for crash safety, the g-forces in a simulator are minimal, and counterweights may be used to offset the head-supported weight. In contrast, the weight and center of gravity (`CG`) requirements for operational HMDs may require much smaller and lighter systems.
In addition, future aviation requirements will impose severe competition for the area about the pilot's head and face. The helmet shield will have to provide increased impact, penetration and ballistic protection. The pilot will also have to wear protection from laser and flashblindness threats, and he may also have to wear a complete nuclear/biological/chemical protective suit. Some provision for head tracking may also be necessary, as well as a provision for night vision goggles to augment synthetic imagery. These headborne equipment requirements complicate the task of an HMD designer in terms of size, weight, and CG requirements.
It must be remembered that the helmet's primary purpose is life-support. An HMD should only be added if it does not compromise the protective capabilities of the helmet. Thus, although an HMD with a wide FOV is desirable, it cannot simultaneously endanger the pilot.